1. Field of the Invention
This invention relates to an optical-recording medium on which information can be recorded, an information-recording apparatus that can record information on that optical-recording medium, and an information-reproduction apparatus that can reproduce information from that optical-recording medium.
2. Related Art
Among optical-recording media such as CD-ROM (Compact Disc-Read Only Memory), CR-R (Compact Disc-Recordable), DVD-ROM, DVD-R and the like, development is being carried out for optical-recording media such as a multi-layer or dual-layer optical disc made by layering or applying a plurality of recording layers on top of the same substrate. Also, in an information-recording apparatus such as a DVD recorder that records on this kind of dual-layer type disc, or in other words, a two-layer type optical disc, by focusing a laser beam for recording on the recording layer (called the ‘L0 layer’ in this specification) that is located on the very front as seen from the irradiation side of the laser beam (in other words, the side near the optical pick up), information is recorded onto the L0 layer by adding heat or the like using a method such as an irreversible-change recording method or re-writable method, and by focusing that laser beam onto a recording layer (called the ‘L1 layer’ in this specification) that is located at the back side of the L0 layer as seen from the irradiation side of the laser beam (in other words, the side farthest from the optical pickup) by way of the L0 layer or the like, information is recorded onto the L1 layer by adding heat or the like using a method such as an irreversible-change recording method or re-writable method.
Also, in the case of a multi-layer optical disc, the L0 layer must let light pass through it, so the reflection film becomes an extremely thin film, however, the L1 layer must not let light pass through it, so in comparison with the L0 layer, there are no limits on the thickness of the reflection layer. By doing this, in the optical and thermal design, there is a disadvantage in that the margin of the L0 layer is narrow. Regardless of this, conventionally, recording has been performed on both layers using the same recording capacity, so differences occur in the recording characteristics of recording on the L0 layer and L1 layer, and particularly, this difference becomes an inconvenience the higher the linear velocity becomes.
For example, of the recording characteristics of each of the layers of a 2-layer DVD-R disc on which information has been recorded at 8× speed, the degree of modulation and the reflectivity are nearly the same, however, the jitter characteristic for the L0 layer is greater than 8%, whereas, for the L1 layer, it is 7%, which is 1% less than the standard 8% for a DVD disc. Therefore, conventionally, in the case of recording on a multi-layer optical disc at high linear velocity, as in the invention disclosed in Reference 1, there is a method for changing the recording capacity for the L0 layer and L1 layer. More specifically, without changing the overall recording capacity of the entire optical disc, the recording capacity of the L1 layer, which has more leeway in the recording characteristics, is increased, and the recording capacity of the L0 layer is decreased, so that the jitter characteristic is about the same for each layer. (Reference 1: Japanese Laid Open Patent Application No. 2001-14677)
Also, in the case of a 2-layer DVD-R disc, when recording information onto the L1 layer, information must have been recorded at the same radial location of the L0 layer as the L1 layer being recorded on. This is because, when recording on the L1 layer in any part where information is not recorded at the same radial location on the L0 layer, the recording sensitivity of the L1 layer will change due to changes in the transmittance of the L0 layer below it, and thus variations in recording characteristics occur. However, since bonding misalignment and eccentricity occur in the optical disc, when recording information onto the L1 layer starting from the same address where information has already been recorded on the L0 layer, there is an inconvenience in that information will be recorded on the L1 layer in a location where information is not recorded on the L0 layer. Therefore, in order to comply with DVD standards, the address position on the L1 layer that corresponds to the same address on the L0 layer is offset about 105 μm further on the inside than the position on the L0 layer, and the inconvenience due to bonding misalignment and eccentricity is avoided. (Reference 2: Japanese Laid Open Patent Application No. 2000-311346) (Reference 3: Japanese Laid Open Patent Application No. 2001-23237)
In order to make the jitter characteristic of each layer of the optical disc about the same without changing the overall recording capacity of the entire optical disc by increasing the recording capacity of the L1 layer and decreasing the recording capacity of the L0 layer as in the aforementioned case of the invention disclosed in Reference 1, there are two methods; or more specifically, a method of changing the linear velocity for each layer without changing the transfer rate; or a method of changing the transfer rate for each layer without changing the linear velocity.
In both of these two methods, either the linear velocity must be changed or the transfer rate must be changed, so there is an inconvenience in that it is necessary to change the control system of the recording apparatus.
Also, a DVD is recorded at CLV (Constant Linear Velocity), so compared with the outer circumference, a longer radial distance is covered on the inner circumference in order move over the same address. Therefore, in order to reach an address position on the L1 layer that is the same as an address position on the L0 layer, it is necessary in the case of the L1 layer to move further toward the inside than in the case of the L0 layer, so by offsetting the address position 105 μm on the outer side, the amount of offset on the inner side becomes about 260 μm, so there is no space on the inner side for an area for OPC (Optical Power Control), or for an area for recording RMD (Recording Management Data). A method is feasible in which the drive calculates the relationship between the address and radius by referencing a data table, however, there is an inconvenience in that the operation becomes more complicated.